In microbiology, McFarland standards are used as a reference to adjust the turbidity of bacterial suspensions so that the number of bacteria will be within a given range.
Original McFarland standards were mixing specified amounts of barium chloride and sulfuric acid together. Mixing the two compounds forms a barium sulfate precipitate, which causes turbidity in the solution. A 0.5 McFarland standard is prepared by mixing 0.05 mL of 1.175% barium chloride dihydrate (BaCl2•2H2O), with 9.95 mL of 1% sulfuric acid (H2SO4).
Now there are McFarland standards prepared from suspensions of latex particles, which lengthens the shelf life and stability of the suspensions. The standard can be compared visually to a suspension of bacteria in sterile saline or nutrient broth. If the bacterial suspension is too turbid, it can be diluted with more diluent. If the suspension is not turbid enough, more bacteria can be added.
McFarland Nephelometer Standards:
| McFarland Standard No. | 0.5 | 1 | 2 | 3 | 4 |
| 1.0% Barium chloride (ml) | 0.05 | 0.1 | 0.2 | 0.3 | 0.4 |
| 1.0% Sulfuric acid (ml) | 9.95 | 9.9 | 9.8 | 9.7 | 9.6 |
| Approx. cell density (1X10^8 CFU/mL) | 1.5 | 3.0 | 6.0 | 9.0 | 12.0 |
| % Transmittance* | 74.3 | 55.6 | 35.6 | 26.4 | 21.5 |
| Absorbance* | 0.132 | 0.257 | 0.451 | 0.582 | 0.669 |
*at wavelength of 600 nm
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“Measured by any standard known to science—by horse-power, calories, volts, mass in any shape,—the tension and vibration and volume and so-called progression of society were full a thousand times greater in 1900 than in 1800;Mthe force had doubled ten times over, and the speed, when measured by electrical standards as in telegraphy, approached infinity, and had annihilated both space and time. No law of material movement applied to it.”
—Henry Brooks Adams (1838–1918)